Whether invertebrates can feel pain is a contentious issue. Although there are numerous definitions of pain, almost all involve two key components. First, nociception is required. This is the ability to detect noxious stimuli which evokes a reflex response that moves the entire animal, or the affected part of its body, away from the source of the stimulus. The concept of nociception does not necessarily imply any adverse, subjective feeling; it is a reflex action. The second component is the experience of "pain" itself, or suffering— i.e., the internal, emotional interpretation of the nociceptive experience. Pain is therefore a private, emotional experience. Pain cannot be directly measured in other animals, including other humans; responses to putatively painful stimuli can be measured, but not the experience itself. To address this problem when assessing the capacity of other species to experience pain, argument-by-analogy is used. This is based on the principle that if a non-human animal's responses to stimuli are similar to those of humans, it is likely to have had an analogous experience. It has been argued that if a pin is stuck in a chimpanzee's finger and they rapidly withdraw their hand, then argument-by-analogy implies that like humans, they felt pain. It has been questioned why the inference does not then follow that a cockroach experiences pain when it writhes after being stuck with a pin. This argument-by-analogy approach to the concept of pain in invertebrates has been followed by others. The ability to experience nociception has been subject to natural selection and offers the advantage of reducing further harm to the organism. While it might be expected therefore that nociception is widespread and robust, nociception varies across species. For example, the chemical capsaicin is commonly used as a noxious stimulus in experiments with mammals. However, Heterocephalus glaber, the African naked mole-rat, is an unusual rodent species as it shows a remarkable lack of pain-related behaviours to capsaicin. Birds, for instance, are unaffected by capsaicin because their capsaicin receptor proteins do not respond to it, but what makes the African naked mole rat unique is that, while it is indeed a mammal, and capsaicin receptor proteins in mammals are indeed very sensitive to capsaicin, the African naked mole rat lacks pain-related neuropeptides such as substance P in its cutaneous sensory fibres. Similarly, capsaicin does trigger nociceptors in some invertebrates, but this substance is not noxious to Drosophila melanogaster (the common fruit fly). Criteria that may indicate a potential for experiencing pain include:
Has a suitable nervous system and receptors Physiological changes to noxious stimuli Displays protective motor reactions that might include reduced use of an affected area such as limping, rubbing, holding or autotomy Has opioid receptors and shows reduced responses to noxious stimuli when given analgesics and local anaesthetics Shows trade-offs between stimulus avoidance and other motivational requirements Shows avoidance learning Exhibits high cognitive ability
Suitable nervous system
Central nervous system Brain size does not necessarily equate to complexity of function. Moreover, weight for body-weight, the cephalopod brain is in the same size bracket as the vertebrate brain, smaller than that of birds and mammals, but as big or bigger than most fish brains.
Invertebrate nervous systems are very unlike those of vertebrates and this dissimilarity has sometimes been used to reject the possibility of a pain experience in invertebrates. In humans, the neocortex of the brain has a central role in pain and it has been argued that any species lacking this structure will therefore be incapable of feeling pain. However, it is possible that different structures may be involved in the pain experience of other animals in the way that, for example, crustacean decapods have vision despite lacking a human visual cortex.
Two groups of invertebrates have notably complex brains: arthropods (insects, crustaceans, arachnids, and others) and modern cephalopods (octopuses, squid, cuttlefish) and other molluscs. The brains of arthropods and cephalopods arise from twin parallel nerve cords that extend through the body of the animal. Arthropods have a central brain with three divisions and large optic lobes behind each eye for visual processing. The brains of the modern cephalopods in particular are highly developed, comparable in complexity to the brains of some vertebrates (see: Invertebrate brain). Emerging results suggest that a convergent evolutionary process has led to the selection of vertebrate-like neural organization and activity-dependent long-term synaptic plasticity in these invertebrates. Cephalopods stand out by having a central nervous system that shares prime electrophysiological and neuroanatomical features with vertebrates like no other invertebrate taxon.
Nociceptors
Nociceptors are sensory receptors that respond to potentially damaging stimuli by sending nerve signals to the brain. Although these neurons in invertebrates may have different pathways and relationships to the central nervous system than mammalian nociceptors, nociceptive neurons in invertebrates often fire in response to similar stimuli as mammals, such as high temperature (40 °C or more), low pH, capsaicin, and tissue damage. The first invertebrate in which a nociceptive cell was identified was the medicinal leech, Hirudo medicinalis, which has the characteristic segmented body of an Annelida, each segment possessing a ganglion containing the T (touch), P (pressure) and N (noxious) cells. Later studies on the responses of leech neurones to mechanical, chemical and thermal stimulation motivated researchers to write "These properties are typical of mammalian polymodal nociceptors".
… excerpt ends here. Continue reading the full article.
